Copper arsenate finds application in wood preservatives such as chromated copper arsenate. Chromated copper arsenate is generally made by dissolving the oxides of the constituent elements or by dissolving copper arsenate in chromic acid. Copper arsenate could be prepared by reacting solutions of a soluble arsenate and copper sulfate that are obtained from metallurgical processes that include leaching of arsenic- containing materials and copper-containing materials.
Processes for making copper arsenate have been disclosed such as in Canadian Patents 1 094 973 and 1 029 298, German Offen. DE 3 048 404 and U.S. Pat. No. 4 405 569.
The copper arsenate prepared from such processes has, generally, too high an impurity content to be acceptable under new specifications that require the total lead and iron content to be less than 0.1% by weight.
A method for the preparation of a low impurity-containing copper arsenate that meets the specifications has been disclosed in co-pending U.S. Patent Application No. 239 897 assigned to the same assignee as the instant invention. According to this method, a solution of a soluble arsenate is added to copper sulfate solution, either or both solutions containing sludge-forming impurities, to form a copper and arsenic-containing solution without precipitating copper arsenate but precipitating the impurities by controlling the pH. After removal of precipitated impurities, the solution is neutralized to precipitate copper arsenate, and copper arsenate substantially free of impurities is recovered.
Although the impurity content of the copper arsenate product from this process is within the specifications, the method has the serious disadvantage that the water content of the copper arsenate is too high, usually 20% or higher. The high water content adds to transportation costs or requires additional processing to reduce it to an acceptable level of less than about 15%. The water can be removed by drying, but this is expensive and causes hygiene problems in the work place.